From the NWS:
An organized cluster of thunderstorms will be located roughly from northeast Iowa into southwest Wisconsin after sunset this evening. This cluster of storms will likely be starting a weakening trend as it approaches far northern Illinois in the mid to late evening hours. Showers and embedded thunderstorms will then gradually shift southeast across northern Illinois and possibly into northwest Indiana through the overnight.
Lightning strikes, gusty winds and downpours will be the main threats, with the highest potential for stronger wind gusts favored to be in the light brown shaded area on the map.